Polymer dispersants have been known to be effective in dispersing pigment in nonaqueous solvent, and such nonaqueous pigment dispersant have been used in producing pigment dispersion compositions for preparation of solvent-based paint compositions. Nonaqueous pigment dispersants have been used widely, for example, as solvent paints for automobile exterior and inks for the color filters of liquid crystal displays. The color filter is a colored pattern formed on a transparent substrate by mixing a pigment dispersion and a light-sensitive composition, coating the mixture, and exposing the formed film to light. In the field of color filter, along with recent improvement in quality of liquid crystal displays, there is an increasing demand for higher color saturation and brightness and accordingly for finer pulverization of the pigment in color filter and increase in the content of the pigment.
Graft polymers are known to be particularly superior in properties as the polymers for such dispersants. For example, JP-A No. 10-339949 discloses a pigment dispersion composition containing a copolymer of an oligomer having an ethylenic unsaturated double bond and a monomer having a nitrogen atom-containing group and an ethylenic unsaturated double bond. Alternatively, JP-A No. 2003-517063 discloses a composition containing a graft polymer composition having amide functional groups in the main chain as the groups for adsorbing pigments.